This application claims priority to and the benefit of Australian Provisional Application Number PQ8180, which was filed in Australia on Jun. 15, 2000.
This invention relates to the casting of metal strip and making of cast steel strip. It has particular application to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term xe2x80x9cnipxe2x80x9d is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip so as to form a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
The setting up and adjustment of the casting rolls in a twin roll caster is a significant problem. The casting rolls must be accurately set to properly define an appropriate separation of the casting rolls at the nip, generally of the order of a few millimeters or less, There must also be some means for allowing at least one of the rolls to move outwardly against a biasing force to accommodate fluctuations in strip thickness particularly during start up.
Usually, one of the rolls is mounted in fixed journals, and the other roll in rotatably mounted on supports that can move against the action of biasing means to enable the roll to move laterally to accommodate fluctuations in casting roll separation and strip thickness. The biasing means may be in the form of helical compression springs or alternatively, may comprise a pair of pressure fluid cylinder units.
A strip caster with spring biasing of the laterally moveable roll is disclosed in Australian Patent Application 85185/98 and corresponding U.S. application Ser. No. 09/154213. In that apparatus, the biasing springs act between the roll supports and a pair of thrust reaction structures, the positions of which can be set by operation of a pair of powered mechanical jacks to enable the initial compression of the springs to be adjusted to set initial compression forces which are equal at both ends of the roll. The positions of the roll supports need to be set and subsequently adjusted after commencement of casting so that the gap between the rolls is constant across the width of the nip in order to produce a strip of constant profile. However, as casting continues the profile of the strip will inevitably vary due to eccentricities in the rolls and dynamic changes due to variable heat expansion and other dynamic effects.
Eccentricities in the casting rolls can lead to strip thickness variations along the strip. Such eccentricities can arise either due to machining and assembly of the rolls or due to distortion when the rolls are hot possibly due to non-uniform heat flux distribution. Specifically, each revolution of the casting rolls will produce a pattern of thickness variations dependent on eccentricities in the rolls and this pattern will be repeated for each revolution of the casting rolls. Usually the repeating pattern will be generally sinusoidal, but there may be secondary or subsidiary fluctuations within the generally sinusoidal pattern.
With improvements in the design of the casting rolls for a twin roll caster, particularly by the provision of textured surfaces which enable control of the heat flux at the interface between the casting rolls and the casting pool, it has been possible to achieve dramatic increases in strip casting speeds. However, when casting thin strip at high casting speeds there is an increased tendency to produce both high and low frequency gauge variations.
We have found that the gauge variations in cast strip can be alleviated by reducing the casting roll separation force and that the defect can be practically eliminated if the roll separation force in minimized. In practice there is at least a certain force that is required to balance the hydrostatic pool pressure and to overcome the mechanical friction involved in moving the rolls. We have also found that the high frequency gauge variation can be overcome, and a unique cast steel strip can be produced, by reducing the strip stiffness in the region of the nip by allowing a quantity of mushy or molten metal to be passed through the nip between the two solidified shells of the strip, by maintaining a roll gap at the nip slightly greater than the gap determined by the fully solidified shell thickness. It is desirable for these purposes that the mechanical friction forces involved in movement of the casting rolls relative to each other is minimized. By achieving very low strip stiffness, the dynamic interaction of the rolls on the strip is uncoupled, and consequently periodic gauge variation regeneration can be substantially reduced if not eliminated.
In at least one aspect, the present invention combines the features of applying a constant casting roll separation force (which can be small) and establishing a constant roll gap that will enable molten metal to be passed through the nip to further reduce strip stiffness. In order to maintain the constant separation force together with a constant roll gap, the invention may also allow for roll eccentricity compensation.
According to the invention there is provided a method of casting metal strip including introducing molten metal between a pair of chilled casting rolls forming a nip between them to form a casting pool of molten metal supported on the rolls, confining the pool at the ends of the nip by pool confining closures and rotating the rolls such that shells of metal solidify from the casting pool onto the casting rolls and are brought close together at the nip to produce a solidified strip delivered downwardly from the nip The casting rolls are biased bodily toward each other, in at least some embodiments under a substantially constant biasing force, and are maintained with a substantially constant gap between them at the nip. This gap is such as to maintain separation between the solidified shells at the nip so that molten metal passes in the space between them through the nip and is, at least in part, subsequently solidified between the solidified shells within the strip below the nip.
The molten metal may be molten steel and the method may produce solidified steel strip at a casting speed of at least 30 meters/minute. The casting speed may be at least 60 meters/minute. The separation space between the solidified shells at the nip may be in the range 0 to 50 microns. This separation provides for maintaining a substantially constant gap with a small biasing force
Said biasing force may be substantially equal to or slightly more than the minimum force required to balance the hydrostatic pressure of the casting pool and to overcome the mechanical friction involved in moving the biased roll. For 500 mm rolls 1350 mm wide and 175 mm pool, putting aside mechanical friction that should be kept small, the hydrostatic force of the molten casting pool will be about 0.75 kN. The biasing force, therefore, may be in range 0.75 to 2 kN per chuck (i.e., per side), and the corresponding roll separation force in the range of substantially 0 to 1.25 kN. Roll separation force is the net force exerted on the strip.
The roll biasing force may be in the range of 0.75 to 1.2 kN and the corresponding roll separation force substantially 0 to less than 0.45 kN. For strip thicknesses above 1 mm the roll separation force may be less than 0.45 kN. By way of example for 1.6 mm thick strip the roll separation force is about 0.45 kN.
At least one casting roll may be mounted on a pair of moveable roll supports moveable to provide said bodily movement of at least one of the casting rolls relative to the other casting roll, and said biasing force may be applied to the roll supports by a pair of biasing units. Each biasing unit may includes a thrust generator acting between a thrust transmission structure connected to the respective roll support, and a thrust reaction structure generating a thrust on the roll support dependent on the spacing between the thrust reaction structure and the thrust transmission structure. The thrust generator may comprise a compression spring or pressure fluid cylinder unit.
The described method may then include the steps of initiating casting of the strip with a gap between the rolls determined by having the solidified shells to meet at the nip, allowing said one roll to move bodily to follow strip thickness variation due to casting roll eccentricities to establish a pattern of roll movements due to those eccentricities, applying the same pattern of movement to the thrust reaction structures of the biasing units to maintain a constant biasing force, increasing the gap between the casting rolls such that molten metal passes through the nip between the solidified shells, and continuing casting of the strip with the increased gap held substantially constant and applying said pattern of movement to the thrust reaction structures to maintain a substantially constant roll biasing force.
Further provided is apparatus for continuously casting metal strip comprising a pair of parallel casting rolls forming a nip between them; metal delivery means to deliver molten metal into the nip between the rolls to form a casting pool of molten metal supported on casting roll surfaces immediately above the nip; pool confining means to confine the molten metal in the casting pool against outflow from the ends of the nip; and roll drive to drive the casting rolls in the counter-rotational directions to produce a solidified strip of metal delivered downwardly from the nip; wherein at least one of the casting rolls is mounted on a pair of moveable roll carriers which allow that one roll to move bodily toward and away from the other roll, wherein there is a pair of roll biasing units acting one on each of the pair of moveable roll carriers to bias said one roll bodily toward the other roll, and wherein each roll biasing unit comprises a thrust transmission structure connected to the respective roll carrier, a thrust reaction structure, a thrust generator acting between the thrust reaction structure and the thrust transmission structure to exert a thrust on the thrust transmission structure and the respective roll carrier, thrust reaction structure setting means operable to vary the position of the thrust reaction structure, and control means to control operation of the setting means so as to replicate a pattern of movement of the roll supports due to roll eccentricities as an applied pattern of movements of the thrust reaction structure to maintain a constant roll biasing force, and roll gap control means operable to increase the gap between the rolls after said applied pattern of movements has been established.
The roll gap control means may be operable to produce an incremental increase of the roll gap in the range 0 to 50 microns. The roll gap control means may be operable to move said one roll. Alternatively, it may be operable to move the other casting roll. In other embodiments, to provide small roll separation force, the roll gap may be fixed and the casting speed may be varied until the requisite separation force is achieved. In that case, eccentricity compensation may be applied prior to providing speed adjustment.
The present invention may provide a unique cast steel strip with a composition as described in more detail below in the description of the embodiments described with reference to the drawings.